Brief historical review
Gamma-rays interacting with the Earth’s atmosphere initiate electromagnetic cascades. At sufficiently high energies the number of cascade particles is sufficient to obtain adequate information about the energy, direction and type of primary particles based on the study of spatial and temporal properties of secondary cascade products. Therefore the arrays of particle (electron, muon, hadron) detectors used in the traditional cosmic ray experiments can serve as effective tools also for a search for sources of very high energy Y-rays. In the 1980s, trying to pursue this technique, several cosmic ray groups reported the detection of excess events over the isotropic cosmic ray background from the direction of famous X-ray binaries Cygnus X-3 (see e.g. Samorski and Stamm, 1983; Lloyd-Evans et al., 1983), and Her X-1 (Dingus et al., 1988). Actually, claims of detection of Y-rays from Cygnus X-3 at lower, TeV energies were first made in the mid 1970s (by the Crimean group) and continued through the mid 1980s (by the Whipple, Durham, Haleakala and some other groups). This controversial episode in gamma-ray astronomy is described in a review article by Weekes (1992).
These exciting reports initiated new air-shower arrays specifically designed for Y-ray studies, in particular the CASA-MIA (Borione et al., 1994), and HEGRA (Karle et al., 1995) detectors with significantly improved sensitivities, lower energy thresholds, and relatively effective hadron/Y separation capabilities. The all-sky surveys by these detectors did not, however, reveal point sources of Y-rays (Cronin et al., 1993) down to flux levels of ~ 10-14 ph/cm2s above 100 TeV. To a certain extent, this cannot be interpreted as a big surprise. The production of such energetic photons requires charged parent particles of energy exceeding several times ~ 1014 eV. Although the spectrum of cosmic rays extends up to 1020 eV, it is quite possible that the acceleration efficiency of protons in the galactic sources, in particular in SNRs, drops at 100 TeV/amu.Thus even in the presence of dense target material, the n0-decay Y-ray emission above 10 TeV is expected to be strongly suppressed. The problem of the high-energy cutoff exists, actually even more seriously, also for the second important channel of Y-ray production through the inverse Compton scattering. The severe synchrotron losses and reduction of the cross-section of the Compton scattering due to the Klein-Nishina effect make this mechanism at such high energies much less efficient than in the TeV region.
From this point of view, nonthermal extragalactic sources like the jets in powerful radiogalaxies and quasars, and the rich galaxy clusters that can accelerate protons well beyond 1015 eV, are certainly more promising objects for 100 TeV observations. But, unfortunately, because of absorption in the extragalactic radiation fields, only a small part of the local intergalactic space within a few Mpc is transparent for > 100 TeV Y-rays.
Thus, by reduction of the energy threshold of detection methods down to 10 TeV and below one may hope to boost the chances of discovery of VHE Y-ray sources both in and beyond of our Galaxy. Such sources have been indeed detected, due to the successful realization of the the so-called Imaging Atmospheric Cherenkov Telescope (IACT) technique.
A remarkable feature of this technique is its high detection rate capability, a consequence of the large integration area of air showers. Even a simple device consisting of a fast (nanosecond) detector of optical radiation (a photomultiplier) in the focal plane of a modest both in quality and size (« 10 m2) reflector, can provide huge, as large as 3 x 108 m2, area for detection of 1 TeV Y-rays. However, the goal cannot be achieved without an effective method of suppression of several heavy backgrounds of different origin. For example, such a simple device cannot distinguish between electromagnetic and hadronic showers, and thus works also as an effective collector of cosmic rays, the flux of which exceeds by several orders of magnitude the flux of Y-rays. This obviously limits the sensitivity of the instrument as a Y-ray detector. Another background caused by the integrated light from the night-sky, limits the minimum detectable energy of Y-rays. Fortunately, the imaging technique provides an adequate background rejection power (see below). The reported TeV Y-ray signals from more than 10 astrophysical objects by several instruments installed in both the northern (Whipple, HEGRA, CAT, Telescope Array, CrAO, SHALON, TACTIC) and southern (CANGAROO, Durham) hemispheres basically proved the early theoretical predictions (Hillas, 1995) concerning the potential of this technique.
The first Cherenkov light pulses from atmospheric air showers were registered by Galbraith and Jelley in 1953. The attempt to pursue the detection of Y-rays from astrophysical sources with the first atmospheric Cerenkov telescopes (Jelley and Porter, 1963, Chudakov et al., 1965) resulted in meaningful upper limits that appeared below the optimistic theoretical predictions. Several years later the first positive signal was reported from the Crab Nebula. The result was obtained with a 10-meter-diameter Cherenkov telescope completed in 1968 at Mt. Hopkins in southern Arizona. In contrast to the high mechanical and optical qualities of this telescope, which after more than 30 years still remains one of the best in the field, the focal plane instrumentation was relatively primitive, thus only a marginal signal at a level of « 3a was revealed after 150 hours of observations accumulated during 1969-1972 (Fazio et al., 1972).
For the next 10 years or so the field languished. The activity in ground based observations significantly declined, and the interest was shifted to two successful satellite-based experiments, SAS-II and COS B, that opened up the observational gamma-ray astronomy at energies above 100 MeV. But in the mid 1980s the interest in ground-based observation turned back, motivated basically by the above mentioned claimed of unusual signals from Cygnus X-3 and some other X-ray binaries (for a review see Weekes, 1992). Even more astonishment was introduced by the claims of periodic signals from Cyg X-3 by underground experiments originally designed for searches for proton decays. However, despite the number of claimed detections, each individual result did not exceed a few-standard-deviations significance, and as observations improved in sensitivity, the signal from Cyg X-3 appeared to diminish proportionally.
It should be noticed, however, that this disappointing episode in the history of gamma-ray astronomy had also positive impacts. Since the unusual "signals" from Cyg X-3 could not be explained in the framework of conventional physics, it attracted many experienced specialists from other fields, in particular from the high energy physicists community, and initiated a new research area called Astroparticle Physics – currently a very popular discipline, albeit with somewhat different (from the 1980s) emphasis on the potential topics and priorities.
Also, as the interest in confirming the existence of TeV signals from Cygnus X-3 began to peak, the Crimean group led by A.A. Stepanian and the Whipple group led by T.C. Weekes started practical steps in the direction of improving the sensitivity of the Cherenkov telescopes by implementing the imaging technique. The idea was that the analysis of the angular distribution of the Cherenkov radiation of air-showers should allow a significant reduction of the cosmic ray background. Hillas (1985) clearly demonstrated that indeed the analysis of the second moments of the Cherenkov images of air showers – as detected by a high quality mirror with a multi-channel imaging camera at its focus – should be able to discriminate between the Y-ray and proton- induced showers, and thus to improve significantly the signal-to-noise ratio. The exploitation of this technique by the Whipple telescope equipped with a 37-photomultiplier camera resulted in the first high-confidence detection of TeV Y-rays from an astrophysical object – a 9a Y-ray signal from the Crab Nebula (Weekes et al., 1989). The construction of a new 109-channel camera, as well as subsequent improvements in the data analysis technique soon led to new important discoveries with this telescope – the detection of Y-rays from Mkn 421 (Punch et al., al. 1992) and Mkn 501 (Quinn et al., 1996).
With arrival of several new projects in the mid 1990s - the CANGA-ROO 3.8m, Durham (both in Australia), CAT in the French Pyrenees, the HEGRA telescope system on the Canary Island La Palma, the Telescope Array in Utah (USA), G-48 in Crimea and some others, vigorous activity commenced with a hope to increase significantly the number of TeV Y-ray sources. Perhaps one may conclude that, in the sense of number of discovered objects, the hope has been only partly fulfilled. Presently, 6 to 8 objects are firmly established as Y-ray emitters, whereas another 10 or so are considered as likely candidates. However, the achievements of the field, in particular the astrophysical significance of the reported results, cannot be reduced to the number of detected sources. For example, the recent comprehensive studies of spectral and temporal characteristic of TeV emission of Mkn 421 and Mkn 501 by the CAT, HEGRA and Whipple IACTs on timescales down to 1 hour, yielded perhaps the highest experimental quality achieved in gamma-ray astronomy, including the MeV and GeV bands. It is difficult to overestimate the significance of these observations for the current models of nonthermal processes in the relativistic jets of blazars. The same is true for TeV Y-ray emission reported from three shell type supernova remnants, SN 1006, RX J1713.7-3946 (CANGAROO) and Cas A (HEGRA), for understanding of the origin of galactic cosmic rays. Several TeV sources have been detected by the HEGRA stereoscopic telescope system at the flux level as small as 10-12 erg/cm2s, and localised within a few arcminutes. This is a remarkable accomplishment that can be achieved at MeV/GeV energies only with the next generation satellite-borne instruments like GLAST. The number of TeV sources is growing rather fast, and hopefully many more sources will be found with the forthcoming IACT arrays CANGAROO-III, H.E.S.S., MAGIC and VERITAS. These detectors will operate at thresholds around 100 GeV, and provide flux sensitivity at TeV energies down to 10-13 erg/cm2s. This should lead, hopefully in the near future, to a dramatic growth in the number of VHE sources.
Reported TeV sources
The sources reported as TeV Y-ray emitters by different groups are shown in Fig. 1.1. They are referred to two categories of detection – "confirmed" (by the same or an independent group) and "not confirmed". Actually, such a division is rather conditional, and does not fully describe the ambiguity of conclusions concerning the confidence level of the reported results. Some of these sources are detected with very high, 10a or more, statistical significance and are confirmed by at least two independent groups. Some others have been detected at high confidence level, e.g. with more than 6a significance, although only by a single group. Finally, several reports claiming detection of new TeV sources have not been confirmed by follow-up observations by the same or by other groups. Currently, the latter is considered as a key condition to ensure membership of the "VHE Source Club". Although generally well justified, this robust condition should be applied cautiously, especially when it concerns a priori or suspected variable objects. On the other hand, one should not overemphasise the claimed very high statistical significance of some detections, because sometimes the large "sigmas" are obtained after optimisation of the image parameter cuts. Also, in some cases the systematic effects are neglected, although they in fact may dominate over the statistical uncertainties. The necessity of independent observations in such cases cannot be questioned.
The Crab Nebula
The Crab Nebula, one of the most prominent objects in the sky, is a unique particle accelerator. The undisputed synchrotron nature of the non-thermal radiation from radio to low energy Y-rays (see Fig. 2.7) indicates the existence of relativistic electrons of energies up to 1016 eV. Given the large magnetic field in the nebula, B > 100 ^G, this implies an extremely effective acceleration at a rate quite close to the maximum possible rate allowed by classical electrodynamics. The Compton scattering of the same electrons leads to effective TeV Y-ray emission.
Fig. 2.7 Nonthermal radiation of the Crab Nebula from radio to very high energy 7-rays. The solid and dashed curves correspond to the synchrotron and inverse Compton components of radiation, respectively, calculated in the framework of the spherically symmetric MHD wind model. The vertical arrows indicated the ranges of characteristic frequencies of synchrotron photons emitted by electrons of different energies.
Despite different approaches and accuracies of calculations performed by many authors in the past, the Crab Nebula was confidently predicted as a strong VHE Y-ray source. Since the first positive report by the Whipple collaboration (Weekes et al., 1999), the Crab Nebula has been detected by more than 10 independent groups using different ground-based techniques. Presently the reported fluxes cover a very broad energy range that extends from 60 GeV (de Naurois et al., 2002) to 20 TeV or even to higher energies.
As the brightest persistent TeV source seen effectively from both hemispheres, the Crab Nebula has become the standard candle for cross-calibration of different detectors. Currently, this is often treated as the most important aspect of ground-based Y-ray observations of Crab, assuming that the "astrophysical" objectives are already achieved, given the good agreement between the reported fluxes and the theoretical predictions (see Fig. 2.7). However, many details remain unresolved and should be addressed by future observations with significantly improved performance in the entire Y-ray domain.
Probing the magnetic fields and electrons. The most informative frequency band to probe the acceleration site(s) and the character of propagation of electrons is the X-ray domain. Chandra, with its sub-arcsecond imaging capability and excellent spectral resolution, is an ideal instrument for such studies. However, the synchrotron data alone tell us only about the product of the magnetic field strength and the density of relativistic electrons. These parameters can be disentangled using additional information contained in Y-rays. Since the TeV Y-rays are produced by IC scattering of electrons responsible also for the observed X-rays, an estimate of the magnetic field based on keV/TeV data concerns the central r ~ 0.5 pc region of the nebula. This corresponds to less than 1 arcmin angular size of the region surrounding the central pulsar. The HEGRA collaboration using its stereoscopic system of 5 IACTs with an angular resolution « 0.1°, set an interesting limit on the angular size of the TeV emission of about 1.5 arcmin, confirming that the TeV y-rays indeed originate in the central part of the nebula. But unfortunately this constraint is not yet sufficient for more definite conclusions about the standard wind termination shock model. The accuracy of the determination of arrival direction of individual Y-rays by future stereoscopic "100 GeV" threshold arrays in the high energy (TeV) domain is expected to be better than a few arcmin. This, combined with large TeV photon statistics, may allow, hopefully, an adequate mapping of the source on < 1 arcmin scales, and thus provide an accurate estimate of the magnetic field in the most interesting sub-pc part of the central region of the nebula.
In all models of the Crab Nebula the calculations of IC fluxes are "controlled" by the observed X-ray flux. The X-ray emission of the Crab Nebula has a distinct axisymmetrical structure (see Fig. 6.14). This is strong evidence that most of the rotational energy of the pulsar is released in the form of a wind which flows out from the pulsar equator. It is natural to expect that the observed TeV fluxes are produced in the same region of X-torus. However, it is difficult to avoid a suspicion that the real picture is more complex. In particular, if the B-field in this region is significantly enhanced and exceeds 0.2 mG, the Y-radiation from the X-torus cannot explain the observed TeV flux. If so, an alternative site for Y-ray production could be regions outside of the X-ray torus, i.e. the parts of the nebula powered by a possible quasi-spherical component of the wind. Although the energy budget of this component of the wind should be significantly less than the luminosity of the equatorial wind, the observed flux of TeV radiation can be achieved assuming a smaller magnetic field in this region. The existence of non-equatorial outflow from the pulsar can be examined by future detailed spatial and spectrometric studies in the X- and TeV Y-ray regimes.
Within the IC models of Y-radiation, the magnetic field in outer parts of the (optical) nebula can be best probed by sub-100 GeV IACT arrays in the northern hemisphere. The angular resolution of the VERITAS telescope array at the threshold of 50-100 GeV is expected to be close to 0.1° which should be sufficient for extraction of such important information. The flux sensitivity and angular resolution of GLAST can provide a complementary study at lower energies (see Fig. 1.2). And finally, the Y-ray fluxes above E > 10 TeV combined with hard X-ray/low energy Y-ray data, should allow determination of the magnetic field in the vicinity of the wind shock front at r ~ 0.1 pc. This compact region cannot be spatially resolved by Y-ray instruments. Nevertheless an indirect "identification" of this region could be possible by detection of time variability of the highest energy tail of Y-ray spectrum on timescales of several months, expected because of the unsteady structure of the shock and rapid synchrotron losses of 100 TeV electrons.
Searching for gamma-rays of "hadronic" origin. The shape of the IC spectrum is rather stable to the basic parameters of the nebula, and can be predicted with high confidence. While at GeV energies the IC spectrum is very hard with a power-law indexin the VHE region the spectrum gradually steepens from
toThis behaviour, which implies an almost constant slope between 1 and 10 TeV, but significant flattening around 100 GeV, is in general agreement with high energy results, and with the recently measured flux at 60 GeV (de Naurois et al., 2002) by CELESTE – a Cherenkov-wave-front detector using a former solar plant at the Themis site in the French Pyrenees (see Fig. 1.2). But for a final conclusion, detailed spectral measurements in the transition region around 100 GeV are needed. It can be done by VERITAS and MAGIC at energies around 100 GeV, and by GLAST at lower energies. Another crucial test can be provided by precise spectrometric measurements at the highest energies well above 10 TeV. In particular, the confirmation of relatively flat, <x E-2’6 type, spectrum extending well beyond 20 TeV as reported by the CANGAROO (Tanimori et al., 1998a) and HEGRA (Horns et al., 2003) groups, would perhaps require an additional radiation mechanism. The Y-rays of n0 origin seem to be an interesting possibility.Because of the limited energy budget determined by the spin-down luminosity of the pulsar, this hypothesis requires an ambient gas density neff in the Y-ray production region exceeding by an order of magnitude the average density of the nebula, n « 5 cm-3. This seems quite unlikely, however cannot not be excluded, e.g. due to possible effective confinement of protons in dense filaments. If so, this should unavoidably result also in an enhanced contribution from electron bremsstrahlung, and, as a consequence, in a noticeably higher Y-ray flux at 100 GeV compared to the pure IC flux. The recent CELESTE measurement of a relatively low flux at 60 GeV does not support this hypothesis, but further studies are needed for a final conclusion.
Searching for gamma-ray signatures of the unshocked wind. The Crab Nebula and other plerions are powered through the termination shocks of the cold ultrarelativistic electron-positron wind with a bulk motion Lorentz factor as large as r ~ 106. It is believed that the region between the pulsar magnetosphere and the shock, where almost all the rotational energy of the pulsar is somehow released in the form of kinetic energy of the wind, is invisible – despite the large Lorentz factor, the wind electrons move together with the magnetic field and thus do not emit synchrotron radiation. Even so, the wind can be directly observed through the bulk motion Comptonization caused by illumination of the wind.This radiation has distinct spectral characteristics depending essentially on the position of the "birthplace" of the particle dominated wind (i.e. the site where the Poynting flux dominated wind undergoes to the regime dominated by the kinetic energy of particles), as well as on the Lorentz-factor and the geometry of propagation of the wind. Thus, dedicated searches for such specific radiation components in the Crab spectrum may provide unique (not accessible at other wavelengths) information about the unshocked pulsar wind. In particular, they can "localise" the region of formation of the particle dominated wind, and "measure" the Lorentz factor of the wind.
The existence of the bright synchrotron nebula around the Crab pulsar very often is interpreted as a crucial condition for effective production of IC Y-rays. In fact, the strong magnetic field in the Crab Nebula produced by the strong wind only reduces the Y-ray production efficiency. Indeed, the energy density of the B-field exceeds by more than two orders of magnitude the radiation density, thus onlyper cent of the energy of accelerated electrons is converted to the IC Y-rays, the rest being emitted in the form of synchrotron X-rays. In other plerions with significantly weaker winds, the resulting nebular B-fields are more than one order of magnitude smaller, which makes these objects more effective Y-ray emitters. The main target photons for inverse Compton scattering in these objects is contributed by the 2.7 K CMBR, therefore the radiative loss of electrons is shared between synchrotron and IC channels asIn a plerion with nebular magnetic field less thanthe Y-ray production efficiency should exceed 1 per cent, given that the cooling time of > 10 TeV electrons (« 1000 yr) is less than the age of typical plerions. This is by an order of magnitude more efficient than the Y-ray production in the Crab Nebula. Correspondingly, one may expect that the next generation IACT arrays with sensitivity better than 10 mCrab should be able to probe TeV Y-ray emission from plerions containing pulsars with the so-called "spin-down" energy flux(see Sec. 6.4).
In fact, three plerions in the Southern Hemisphere – PSR B1706-44, Vela, and PSR B1508-58 – have been already claimed by the CANGAROO group as TeV emitters. While the statistical significance of the signal from PSR B1706-44 detected by the first 3.8m diameter CANGAROO telescope (Kifune et al., 1995) was quite high, and later claimed to be confirmed both by the Durham group (Chadwick et al., 1998a) and by the new 10m CANGAROO telescope (Kushida et al., 2003), the reports on tentative detection of TeV emission from Vela and PSR B1509-58 with the 3.8m CANGAROO telescope are not yet confirmed by other measurements.
Although above we argued that one should expect TeV Y-ray emission from some selected plerions, the reported fluxes of both PSR 1706-44 and Vela are too high to be easily accommodated by the conventional synchrotron-inverse Compton models.
PSR B1706-44. Generally, it is reasonable to assume that the unpulsed TeV radiation from PSR B1706-44 is not directly connected with the 102 ms EGRET pulsar, but rather originates in the surrounding IC nebula with a total TeV Y-ray luminosityBut the problem is that the X-ray luminosity of the region around the pulsar within 1 arcmin is a factor of 3 below the Y-ray luminosity. This is a rather unexpected result, and puts very tight limits on the parameters characterising the TeV Y-ray production region. Indeed, assuming that the X-rays and TeV Y-rays are produced in the same region, we come to the conclusion of an uncomfortably low (for a pulsar wind nebula) magnetic field,A possible way to avoid the problem of such a low B-field is to assume that the electrons occupy a significantly larger region than thearcmin (unresolved by ROSAT) synchrotron X-ray nebula. This could be realized if the electrons quickly leave (e.g. due to the diffusive propagation) the pulsar wind nebula with a conventional, e.g.field, and enter the interstellar medium with much lower field. There they upscatter the 2.7 K CMBR photons and thus produce the bulk of the observed TeV flux.In addition to the high TeV/X-ray flux ratio, the TeV signal of this source contains another puzzle. Surprisingly, the Y-ray production region reported by the CANGAROO group is offset from the Vela pulsar position by about 0.13°. It has been claimed that the position of the Y-ray excess coincides with the supposed "birthplace" of the pulsar as determined from its age and proper motion. The total y-ray luminosity above 1.3 TeV is estimated as
Despite a calm reaction from the gamma-ray community to this result ("why not ?"), the most natural inverse Compton interpretation of the observed TeV emission faces very serious difficulties. The simplest solution would be (from the point of view of a theorist) that both the reported flux and the position of the TeV source are not correct. On the other hand, in the case of confirmation of these results, one would need to invoke certain extraordinary assumptions concerning both the origin of the electrons and the strength of the ambient magnetic field.
The X-ray luminosity of the same region seen by ROSAT and ASCA does not significantly exceed 1032 erg/s (Harding et al., 1997), an order of magnitude less than the TeV luminosity. Thus, in a one-zone synchrotron-Compton model, the magnetic field cannot exceed a few ^G. This implies that the pressure of relativistic electrons in the X-ray hot spot exceeds by two orders of magnitude the pressure of the B-field. Formally we cannot exclude the scenario with an effective escape of electrons, provided that the escape is compensated by continuous particle acceleration. However, since the B-field in the nebula is believed to be much stronger, the escape of electrons should produce an X-ray image with a hole rather than a hot spot in the Y-ray production region. These difficulties of the hypothesis that we are dealing with long-lived particles left in a trail by the pulsar after it moved from its birthplace (Harding et al., 1997), can be somehow tolerated, if we assume that the TeV Y-rays are of hadronic origin, i.e. they are produced by interactions of "relic" energetic protons with the ambient gas.The required total energy in "relic" protons could be reduced down to a reasonable amount ofif we assume that the ambient gas density in this region is significantly higher than outside, For effective confinement of particles in this region with a size ofthe B-field should be close to its equipartition value,i.e.In principle, both the relativistic particles
and the B-field could be created by the powerful relativistic wind of the "baby" pulsar. An alternative source could be the kinetic energy released at the supernova explosion. Obviously, the ‘relic’ electrons could not survive severe synchrotron losses in a such strong magnetic field. At the same time, the X-ray emission of the hot spot still could be explained by the synchrotron radiation of secondaryelectrons and positrons. The energy released in secondary electrons at p-p interactions depends on the spectrum of protons; for hard proton spectra it could be as large as half of the energy transferred toThe secondary TeV electrons quickly lose their energy in the form of synchrotron X-rays. Thus, even within this pure ‘hadronic’ scenario the TeV radiation should be accompanied with non-negligible nonthermal X-ray emission. Unfortunately, the lack of adequate spectral information in X-ray and TeV regions does not allow more quantitative conclusions about the parameters characterising the TeV source.
Gamma ray pulsars
The first realistic attempts of using the atmospheric Cherenkov technique for the detection of VHE Y-ray sources in the late 1960s coincided with the great discovery of pulsars. Since then pulsars have been in the highest priority target lists of many Cherenkov groups, although, as we understand now, this idea does not have a solid theoretical background, and there is no much hope to detect magnetospheric TeV Y-ray emission even from pulsars that are bright in GeV Y-rays. Actually, there were several reports of possible detections of pulsed TeV radiation, in particular from the Crab and Geminga pulsars, however follow-up observations with more sensitive imaging telescopes failed to confirm these early claims. The upper limits of TeV radiation from the MeV/GeV pulsars are shown in Fig. 2.1. These agree with the models which predict intrinsic spectral cutoffs beyond 10 GeV. Irrespective of details of these models, it is likely that flat GeV Y-ray spectra should significantly steepen at higher energies due to attenuation caused by pair production in the pulsar magnetic field. The position of the spectral turnover depends essentially on the localisation of the Y-ray production re-gion(s). Thus, spectrometric measurements at energies above 10 GeV by GLAST, and, possibly, also by very low-threshold (sub-20 GeV) ground based detectors may provide a crucial test for different scenarios of particle acceleration in pulsar magnetospheres.
Meanwhile, the outer magnetosphere gap models predict a new hard component of Y-radiation produced due to the inverse Compton mechanism in the outer magnetosphere.An interesting feature of this radiation is its hard spectrum below 1 TeV, with a sharp cutoff of several TeV. Thus, the most promising energy region for detection of this component seems to be a rather narrow interval around a few TeV (Ro-mani, 1996). In particular, the calculations for the Vela pulsar predict that the energy flux in pulsed TeV Y-rays could exceed 0.1% of the pulsed GeV flux (Romani, 1996). A strong upper limit on the flux from the Vela pulsar reported by the CANGAROO collaboration (Yoshikoshi et al., 1997) at the "right" energies (see Fig. 2.1) is quite close to the predicted flux.